1. Field of the Invention
This invention relates to an index-guided semiconductor laser device which oscillates at a low threshold current.
2. Description of the Prior Art
Conventional semiconductor laser devices are classified into gain-guided semiconductor lasers and index-guided semiconductor lasers. The index-guided semiconductor lasers are advantageous over the gain-guided semiconductor lasers in that they oscillate in a stabilized transverse mode which is important in practical use. BH (buried hetero) lasers and VSIS (V-channeled substrate inner stripe) lasers are typically known as index-guided semiconductor lasers.
FIG. 2(A) shows a BH laser wherein a double-heterostructure is formed in a mesa-fashion on the substrate 1 in a manner to sandwich the active layer 3 for laser oscillation between the cladding layers 2 and 4, and a semiconductor material having a low refractive index is buried in both sides of the mesa-structure, thereby achieving laser oscillation operation based on a complete index-guided action with threshold current of as small as 10 mA or less. However, the BH laser has a disadvantage in that it tends to oscillate in a high-order transverse mode when either of the refractive index of the burying layer 13 made of the low refractive index material or the width w of the waveguide corresponding to the width of the mesa-structure is not selected to be a certain value. Thus, there are many limitations in production conditions. Moreover, the waveguide width w is required to be 2 .mu.m or less to achieve laser oscillation in a fundamental transverse mode, and accordingly the laser facets tend to be broken down even at a relatively low output power, so that neither the mass production of the devices nor the reliability thereof can be maintained. Reference numeral 5 in FIG. 2(A) is a cap layer for achieving ohmic contact with an electrode thereon.
FIG. 2(B) shows a VSIS laser which is fabricated as follows: On a substrate 1, a current blocking layer 6 is formed, followed by the formation of a V-channel which reaches the substrate 1 to form an electroconductive region, and then a double-heterostructure is formed on the current blocking layer 6 including the V-channel in a manner to sandwich a flat active layer 3 between the cladding layers 2 and 4. The VSIS laser is advantageous in that it does not oscillate in a high-order transverse mode even though the waveguide width w corresponding to the width of the V-channel is set to be great, for example, in the range of 4 to 7 .mu.m. That is because light outside of the waveguide is absorbed by the substrate 1 so that gain of the high-order mode can be suppressed. However, the threshold current thereof is in the range of 40 to 50 m .ANG., which is extremely higher than that of the above-mentioned BH laser. That is because carrier injected into the active layer 3 diffuses into both sides of the active layer 3 although current is confined within the inner-striped structure of the current blocking layer 6, and the amount of carriers which are ineffective to laser oscillation increases. FIG. 3 is the carrier distribution showing the carrier density n in the active layer 3 of the VSIS laser in the direction y parallel to the junction. When the waveguide width w is 4 .mu.m, the carriers in the shaded parts are ineffective carriers which do not contribute to laser oscillation. These ineffective carriers are consumed for emitting unnecessary light and for generating heat, causing an increase in the threshold current and a decrease in the reliability of the laser device.